The invention relates to a scanning device for recording and/or play-back of wide-band signals from a magnetic tape by the diagonal track method, including a stationary hollow guide drum having two parts spaced apart to define a slot around which a magnetic tape is wound helically at an angle of less than 360.degree. and a head-wheel carrying at least two magnetic heads is disposed in the drum for rotation of the magnetic heads transversely to the slot for scanning the magnetic tape which is moved diagonally across the slot.
Generally, devices for the recording of sound on magnetic tape utilize a relatively narrow magnetic tape which is guided along one or more stationary magnetic heads. In contrast, magnetic tape recording devices for high frequencies and wide-band signals such as television signals utilize a relatively wide slowly moving magnetic tape which is scanned by one or more magnetic tapes at an angle with respect to the direction of motion of the magnetic tape. Accordingly, the recorded information is retained in the magnetic tape as a series of parallel, finite track segments which extend from one edge of the magnetic tape to the opposite edge at some angle with respect to the edges.
Another known device includes a magnetic tape which moves around a cylinder jacket and is swept by a head wheel in a transversal direction to the direction of motion. Prior art systems include a device in which the magnetic tape is wound helically around a cylindrical guide drum and is scanned in diagonal tracks by a head wheel which revolves in a slot defined by two parts of the drum. The angle of the magnetic tape around the guide drum can lie between 90.degree. and 360.degree. and accordingly, the angular position of the individual, closely adjacent, track segments with respect to the central axis of the magnetic tape can be different. In addition to other requirements, the location and angular position of the track segments must be equivalent for all devices within very close tolerances in order to maintain a good quality of reproduction. This is particularly important for the recording and play-back of television signals.
To achieve a high degree of accuracy, the assemblage of the scanning device must be precise. The assembly of the two parts of the guide drum and the tape guide elements which determine the position of the magnetic tape on the guide drum present the greatest difficulties. The reason for this is that the two parts of the guide drum must individually be accurately dimensioned and their relative position with respect to each other must be precise with regard to the relative displacement and the relative angular positions. Otherwise, the magnetic tape will undergo a change in its geometric shape when passing from one drum section to the other and thereby cause errors in the quality of the operation.
As a result of the high scanning rate and the direct tape-head contact during the recording and play-back of television signals, the magnetic heads in the device undergo considerable wear and require replacement after a few hundred hours of operation. For this reason, it is desirable to have the head wheel arranged for simple replacement after the separation of the parts of the guide drum. In addition, the assembly of the scanning device should be simple and maintain the original precision with respect to positioning.
From the German patent DT-OS 1,930,779, a scanning device of the type mentioned above is known and the method of assembly is also known. That device includes a drum arrangement having two parts with a rotating head wheel arranged therein. The guide drum includes two extremely precise flat surfaces which engage the cylindrical external surface of two parts of the drum arrangement connected with them after an adjustment of the separation of the two parts has been made.
It would be preferable to have a scanning device which permits the head wheel to be replaced by a simple disconnection of the guide drum parts and the simple re-assembly of the guide drum parts without any substantial loss of accuracy.